Name That Warbler

by David Bonter and Irby Lovette

Illustrations by Benjamin Clock

For a bird bander like David Junkin, approaching a mist net is "like going to the mailbox—you never know what you may find." An early morning of bird banding in 2006 turned up a true mystery bird for Junkin, who runs a banding station near his home in Wethersfield, New York, with his wife and research partner, Sandy.

Experienced bird banders are rarely completely stumped by the birds they catch, particularly when the birds are netted in a familiar location. However, on June 27, 2006, David found a unique warbler—one that no one could identify—suspended in the billowing folds of his nylon-mesh mist net. The mystery bird would soon become known informally as the "Junkin's Warbler." Deciphering its true identity would ultimately require the unlikely combination of a feather and a high-tech analysis at the Cornell Lab of Ornithology.

David has been banding birds for 50 years, ever since a high-school biology teacher inspired him to acquire his own license to band birds from the federal government at age 18. Over the intervening years David has banded more than 25,000 birds, ranging from Golden-crowned Kinglets to Herring Gulls. Despite this half-century of experience, David had never before encountered an unknown bird.

The Junkins were banding birds that day as part of the Monitoring Avian Productivity and Survivorship (MAPS) project. MAPS was created by the Institute for Bird Populations in 1989 and now enlists 500 banding stations to help study the survival and reproductive success of more than 120 North American landbirds. Each MAPS station runs a set of 10 mist nets every 10 days during the breeding season. The birds captured in these nearly invisible nylon nets are quickly aged, measured, and sent on their way wearing a lightweight aluminum leg band provided by the federal Bird Banding Laboratory.

As David approached one of his nets at 9:30 A.M. on that summer morning, he knew that he was looking at a bird unlike any he had seen before. Was it a new species for the site? Even upon close inspection in his hand, the warbler's identity remained uncertain. Maybe it was a "Sutton's Warbler"—a rare hybrid between a Northern Parula and a Yellow-throated Warbler. Or was it an even more unusual hybrid, possibly one that had never before been documented?

The Junkins, both experienced birders, consulted various field guides and reference books, but nowhere could they find a description that matched the bird they had caught. David then called Betsy Brooks, a researcher at Braddock Bay Bird Observatory in Rochester, New York, who has banded more than 100,000 birds. They all discussed the field marks of the mystery warbler. Still unable to identify the bird, Betsy confirmed David's fears—if the bird could not be identified, it had to be released without a band, because bands can only legally be placed on birds that are identified. After photographing, measuring, and documenting as much information on the bird as possible, the Junkins released the 13.9-gram mystery and watched it fly off into the forest.

David Junkin holds a mystery songbird he trapped in June 2006. He originally had to release it unbanded, because banding an unidentified bird is illegal. He later received permission from the Bird Banding Laboratory and was able to retrap and band it. Junkin also plucked two tail feathers from the bird for DNA analysis at Cornell. At the bottom of this page are some more commonly seen hybrids: the "Lawrence's" warbler, "Brewster's" warbler, and "Sutton's" warbler. These are not real species but the unusual product of a mating between two distinct species. Sandra Junkin (2)

During the next few days David shared his photographs with local birders, banders, and ornithologists at the Cornell Lab of Ornithology. The pictures were posted on web sites and led to wide speculation about the bird's identity on email lists and birding blogs. Self-described as "computer illiterate," Junkin was stunned by the barrage of interest in his bird. Was the bird a hybrid? Was it an aberrant individual of a common species? With only photographs and measurements to guide speculation, the bird's true identity might never be known.

Intrigued by the report and photographs circulating on the Internet, several of us at the Cornell Lab of Ornithology stayed in touch with David about his mystery bird. Researchers in the Lab's Fuller Evolutionary Biology program had recently compiled a comprehensive evolutionary tree of the warbler family, which required sequencing DNA samples from all warbler species. Unfortunately, identifying the Junkins' mystery warbler using DNA would require a DNA sample from that bird as well.

This led to a plan: if the mystery bird could be recaptured, the DNA extracted from even a single feather could be compared with the Lab's existing database. David and Sandy had a new goal—to recapture the warbler.

Weeks passed with no sign of the mystery warbler, but then on July 13 the Junkins found it hanging in the same net in which it had originally been captured. The second visual inspection did not answer any questions, but the Junkins gently plucked two tail feathers from the bird and sent them via certified mail to the Lab. In the intervening weeks David had received permission from the Bird Banding Laboratory to sample and band the bird, and this time the bird was fitted with band number 2120-86194 (under the ID code "hybrid passerine") before being released. Although it was the peak of the breeding season, David detected no signs that the bird was in breeding condition (the bird lacked both a brood patch and a cloacal protuberance). His impression was that the bird was a male, but he never heard it singing. What would the DNA reveal?

The small feathers David plucked from the bird provided ample DNA. After looking at the photographs making the rounds on the Internet, some people had speculated that the bird was a cross between a Blue-headed Vireo and one of several warbler species, but successful hybridization between such evolutionarily distant families of songbirds is very unlikely. The DNA analysis bore this out and confirmed that the Junkin's Warbler was a hybrid between two warbler species that breed in the Northeast.

By comparing two types of DNA, Lab researchers were able to identify the species of both the bird's mother and father. Identifying the mother was easy, because scientists who study avian evolution rely heavily on the special subset of the bird genome residing in the mitochondrion. This "mitochondrial DNA" is passed only from the mother to her offspring, and its sequence differs substantially even among closely related species. By sequencing the mystery bird's mitochondrial DNA and comparing it with the Lab's database, the mother's identity became instantly clear.

Identifying the father was a bit more difficult, but knowing the mother simplified the process. For this analysis, the genes from the "nuclear genome" of the warbler proved valuable, because (just as in humans) each bird has one copy of these genes inherited from its father and one from its mother. By comparing these nuclear DNA sequences with those of all possible warbler species, the researchers found that one copy matched the database samples of the species already identified as the mother, and the other copy matched the database sequences of a second, different species. This second sequence confirmed the identity of the father.

By now, you are undoubtedly wondering about the identity of this hybrid's parents. Before we tell you our answer, we invite you to examine the illustration and photographs shown here and also the additional photographs posted on the web site www.allaboutbirds.org/mystery and make an educated guess about the parent species based on your own observations of the hybrid bird's characteristics.

Once you have settled on your best guess about the bird's mother and father, you can compare your prediction with the DNA-based answer at the web site mentioned above. We will also present the full results in the Autumn 2007 issue of Living Bird.

Warbler Species That Aren't

Scientists have identified dozens of hybrid crosses between warbler species. Although most combinations are very rare, a few crosses are common enough to be encountered with some regularity. The best known are the hybrids between Blue-winged and Golden-winged warblers, which are depicted in many field guides. Hybrids between these species typically show stereotyped plumage patterns that combine traits from both of the parental species. The most common hybrid form is known as a "Brewster's" warbler; the rarer form is known as a "Lawrence's" warbler. We currently believe that a "Lawrence's" warbler results when two "Brewster's" warblers mate, or when a "Brewster's" warbler backcrosses with one of its parent species, but research into the genetics of hybridization between these species is underway and many questions remain unanswered.

A much less frequently seen hybrid, popularly called the "Sutton's Warbler," is probably a cross between a Northern Parula and a Yellow-throated Warbler.

It is important to remember that these "named" hybrids are not real species, but rather the unusual and probably ephemeral product of a mating between two distinct species. Our use of the term "Junkin's Warbler" follows this popular tradition and is not meant to imply that this hybrid is a new species.

David Bonter is leader of Project FeederWatch in the Lab of Ornithology's Citizen Science program. Irby Lovette is director of the Lab's Fuller Evolutionary Biology program. Benjamin Clock is assisant curator of Visual Media at the Macaulay Library.

For permission to reprint all or part of this article, please contact Tim Gallagher, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-2443. email: twg3@cornell.edu